Amorphous oxide thin films, particularly, transparent amorphous oxide semiconductors, such as In—Ga—Zn—O based (hereinafter, referred to as “IGZO”) semiconductors, are called “TAOSs (Transparent Amorphous Oxide Semiconductors)” and have been studied and developed by various research institutions and makers, and significant technological progress has been made. In particular, with panel makers in Japan, Korea, and Taiwan playing a central role, trial manufacture of large-size displays is actively being carried out, discussions about reliability and stability, and moreover, technical studies toward mass production are also progressing, and production by a vacuum film forming method has almost reached a practical level.
Meanwhile, among the trends toward realizing a larger size, with an aim to reduce production costs, research and development concerning the production of TAOS-TFTs (Thin Film Transistors) by a liquid phase process (hereinafter, referred to as a “liquid phase method”) are also popular. For example, in recent years, a TAOS-TFT-driven liquid crystal display produced by a liquid phase method has been reported, and thus, expectations around the world for the production of TFTs by a liquid phase method have become increasingly higher. However, regarding the liquid phase method, there are problems in that the process temperature is still higher compared to the vacuum film forming method and in that it is difficult to obtain a thin film with good quality, which should be overcome in the future.
Japanese Patent Application Laid-Open (JP-A) No. 2010-258058 discloses a method for producing an amorphous oxide thin film, the method including subjecting a semiconductor precursor film formed on a substrate to a heating treatment together with irradiation with electromagnetic waves, to transform the semiconductor precursor film into an amorphous metal oxide semiconductor film. Further, JP-A No. 2010-258058 also discloses that it is preferable to remove organic components other than metal components by performing washing by a dry washing process such as oxygen plasma or UV ozone washing, after the formation of the semiconductor precursor film but before semiconductor conversion/transform treatment (before heating treatment) to decompose and wash organic substances which exist in the thin film or at the surface of the thin film and cause generation of impurities.